1. Field of the Invention
This invention relates to compositions and processes for forming conversion coatings of novel composition on the surfaces of aluminum and those of its alloys that contain at least 45% by weight, preferably at least 75% by weight, or more preferably at least 90% by weight, of aluminum. The novel conversion coating, particularly when followed by a particular type of organic post-treatment known per se, provides excellent corrosion resistance and adhesion to subsequently applied paints and like protective outer coatings.
2. Statement of Related Art
Conversion coating of aluminum and its alloys generally is well known in the art. Most known and widely used conversion coating processes produce predominantly phosphate and/or chromate microcrystals in the coating, and the coatings contain predominantly divalent or higher valent cations and little or no fluoride or alkali metal ions. If the coatings are predominantly phosphate based, pretreatment with a colloidal titanium phosphate (Jernstedt salt) containing composition is generally required for best results, and the conversion coating composition itself generally contains an accelerator such as nitrite, nitrate, hydroxylamine or one of its derivatives, nitroorganic compounds, chlorate, or the like. These accelerators are often unstable, especially when combined with the other components of a conversion coating composition, so that multiple concentrate packages are often required to furnish all the needed ingredients (other than water) of a complete conversion coating composition, which is sometimes hereinafter called a "bath" for brevity, even though the composition may be satisfactorily brought into contact with the metal to be treated by spraying or other methods as well as by immersion.
Furthermore, for commercially acceptable corrosion resistance in some types of service, phosphate conversion coated metal must be post treated with compositions containing hexavalent chromium in order to "seal" the coating and improve its corrosion resistance. If the conversion coatings themselves are chromate based, the liquid compositions used to form the conversion coatings also normally contain hexavalent chromium, which is now notorious for its pollution of the environment and probable carcinogenicity, and therefore generally may not be legally disposed of without costly pretreatment.